There have been many attempts to develop compact, high precision, low tolerance narrow band optical filters centered at predetermined wavelengths for application in areas such as spectroscopy, optical networks and optical links, and more particularly optical communication systems. Optical filters are some of the most ubiquitous of all passive optical components found in most optical communication systems. One use of optical filters is in the field of optical communications where only a signal of a predetermined wavelength is to be passed.
Narrow band optical filters which pass only a very narrow band of light (e.g. .+-.0.2 nanometers or less) and centered at a predetermined wavelength, are extremely difficult to make and consequently relatively expensive to manufacture. As of late, there has been a demand for a plurality of such filters having a wavelength separation of less than 2 nanometers. One known means for providing a selective narrow band optical filter, is by utilizing a wavelength selective interference filter element whose wavelength characteristic depends on the angle of incidence. Thus, by varying the angle of light incident upon the interference filter, the wavelength of the light that is passed by the filter varies. Such a filter element is described in U.S. Pat. No. 5,331,651 issued Jul. 19, 1994 and assigned to the Hewlett-Packard Company. Often, these filter elements are used in free-space configurations, wherein a beam of light exiting an optical fiber or other waveguide is directed through free space into a wavelength selective interference filter element at a predetermined angle; however, many such configurations have limitations. For example, positioning and affixing an optical fiber in a predetermined position and at a predetermined angle relative to a filter element can be challenging.
Thus, it is an object of this invention, to provide an integrated narrow band-pass filter and method of making a filter, which overcomes many of the limitations in prior art devices, and, wherein the device is compact, centered at a predetermined frequency, and has a tolerance that is within very small predetermined limits.
It is a further object of the invention to provide a method of tuning a filter to obtain a wavelength selective filter that is compact and centered at a predetermined frequency within very small predetermined limits.